Flux for soldering, soldering method, and printed circuit board

ABSTRACT

A flux for soldering used when soldering is performed to a board subjected to electroless nickel plating, the flux containing resin having film forming ability, activator, and solvent, and further containing metallic salt in an amount of 0.1 to 20% by weight of the total amount of flux. The use of this flux suppresses diffusion of nickel solder in soldering portions, and prevents concentration of phosphorous, thereby improving the bonding strength of soldering.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flux for soldering used whensoldering various electronic components on a printed circuit board, andmore particularly to a flux used when soldering is performed onto copperlands of a printed circuit board subjected to electroless nickelplating, as well as a soldering method and a printed circuit board.

2. Description of Related Art

When electronic components are soldered onto a printed circuit board,soldering onto copper lands formed on the printed circuit board hasconventionally been performed with the use of tin/lead alloy solder orlead-free solder. In order to prevent oxidation of copper, electrolessnickel plating is often applied to the surfaces of the lands.

However, when electroless nickel plating is applied to the lands, atrace quantity of phosphorous compound remains in a nickel plating layerbecause hypophosphite is used as reducing agent.

Therefore, during soldering with the use of solder alloy to the surfaceof electroless nickel plating, the nickel in the nickel plating diffusesinto the melted solder alloy, and phosphorus segregates locally at theboundaries between the nickel plating layer and the solder alloy. Thiscauses extremely concentrated portions of phosphorous, and hence bondingstrength is lowered thereby to strip soldering in some cases.

Japanese Unexamined Patent Publication No. 6-264284 discloses a methodof forming a wiring board including: forming electroless nickel/boronplating film on a wiring pattern composed of tungsten or molybdenum andon a connecting pad which are formed on a ceramic printed wiring board;performing heat treatment; and forming a nickel/phosphorous platingfilm. In this method, the drop in bonding strength, due to concentrationof phosphorus in connecting interface during soldering and brazing ofgold/tin, etc., is prevented by setting phosphorus content at not morethan 6%, and a plating film at 0.3 to 2 μm.

Japanese Unexamined Patent Publication No. 10-163404 discloses aninput/output terminal for BGA formed by attaching, instead ofconventional solder balls, Cu balls whose surface is coated with asolder plating layer, to a Cu pad whose surface is coated with anelectroless Au plating layer via a P-containing electroless Ni platinglayer.

These methods require additional processing such as two-stage platingoperation and ball processing. Additionally, the use of specific boardand balls lowers versatility, at which the present invention aims fromthe point of view of performing general solder connection on a generalboard.

SUMMARY OF THE INVENTION

One advantage of the present invention is to provide a flux havingimprovements in bonding strength by suppressing nickel from diffusinginto melted solder alloy during soldering, and preventing concentrationof phosphorous, as well as a soldering method using the flux, and aprinted circuit board.

Other advantage of the present invention is to provide a flux havingimprovements in preservation stability, a soldering method using theflux, and a printed circuit board.

In addition, the above-mentioned problems can be solved simply at lowcost, because there is no need to use specially prepared board andsolder balls.

A flux for soldering of the present invention, which is used whensoldering is performed onto a board subjected to electroless nickelplating, contains resin having film forming ability, activator, andsolvent, and further contains metallic salt, particularly, metallic saltof organic acid, in an amount of 0.1 to 20% by weight of the totalamount of flux.

The metallic salt suppresses the nickel in the lands surfaces fromdiffusing into solder alloy, and prevents phosphorous from concentratingin the boundaries between the lands and the solder. This enables toimprove the bonding strength of the solder. Additionally, the use of theflux of the present invention results in good solderability, and theapplication of the present invention causes no lowering ofsolderability.

The reason why the diffusion of nickel into the solder alloy issuppressed in the present invention is not clear. However, it ispresumed that the metal in the metallic salt of organic acid in the fluxdeposits due to replacement by nickel, and this suppresses the reactionof nickel and the metal in the solder alloy, thereby suppressing thenickel in the lands surfaces from diffusing into the solder alloy.

Preferably, in the present invention, the metallic salt is metallic saltof organic acid, and the activator is organic acid that is the same asorganic acid composing the metallic salt of organic acid, or organicacid having a lower acidity than that. In particular, the flux of thepresent invention preferably does not contain halogen compound such ashydrochloric acid.

By so doing, even if the flux is preserved for a long period of time, itis able to prevent metallic salt from being decomposed, therebyimproving the stability of the metallic salt. Specifically, it is ableto suppress a great drop in the bonding strength of solder and in theinsulation resistance between the lands. Lowering of insulationresistance is liable to occur in, for example, a soldering method inwhich a flux is printed overall on a board having lands, and solderballs are mounted on the lands and allowed to reflow, thereby connectingsolder balls to the lands. Further, the drop in the stability ofmetallic salt can result in insufficient reforming effect of bondingstrength by the addition of metallic salt.

A first soldering method of the present invention is characterized byprinting the above-mentioned flux for soldering on a board having on itssurface copper lands subjected to electroless nickel plating; mountingsolder balls on the lands; and heating to have the solder balls reflowand connect to the lands. Preferably, solder balls to be used arelead-free from the point of view of environmental impact.

A second soldering method of the present invention is characterized byprinting paste that is a mixture of the above-mentioned flux forsoldering and solder powder, on a board having on its surface copperlands subjected to electroless nickel plating; and heating to have thepaste reflow so as to form solder alloy on the lands.

The present invention also intends to provide a printed circuit board inwhich solder is connected by the soldering method as above described.

Meanwhile, a board is also used generally which is obtained by applyingelectroless nickel plating to a board, and further applying plating ofother metal such as gold, in consideration of solderwettability. In thepresent invention, the concept of “a board subjected to electrolessnickel plating” includes a board having plating of other metal, such asgold, on nickel plating.

Other objects and advantages of the present invention will be apparentfrom the following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A flux for soldering of the present invention contains resin having filmforming ability, activator, metallic salt, and solvent. As resin havingfilm forming ability, there are, for example, rosin and thermoplasticacrylic resin.

Acrylic resin has a molecular weight of not more than 10000, preferably3000 to 8000. When the molecular weight exceeds 10000, crackingresistance and debonding resistance may drop. In order to aid activeaction, acid value is preferably not less than 50. Softening point ispreferably 230° C. or below, because it is necessary that acrylic resinis softened during soldering.

Therefore, examples of suitable acrylic resin are composed from monomershaving polymerizing unsaturated group, such as (metha) acrylic acid andester thereof (e.g., methyl (metha) acrylate, etc.), crotonic acid,itaconic acid, maleic anhydride and ester thereof, (metha)acrylonitrile, (metha) acrylamide, vinyl chloride, and vinyl acetate.These are preferably polymerized with the use of catalyst, such asperoxide, by radical polymerization such as bulk polymerization method,solution polymerization method, suspension polymerization method, andemulsion polymerization method.

As rosin, rosins conventionally used in fluxes, and derivatives thereofare usable. As rosin and derivative thereof, general gum, tall, and woodrosin are usable. Examples of the derivative thereof are heat treatedresin, polymerized rosin, hydrogenated rosin, formylated rosin, rosinester, rosin modified maleic resin, rosin modified phenol resin, androsin modified alkyd resin.

The content of resin having film forming ability is in an amount of 20to 80% by weight, preferably, 30 to 65% by weight of the total amount offlux. When the content is less than 20% by weight, wettability maydeteriorate. On the other hand, when the content is over 80% by weight,viscosity control is impossible and hence operability may deteriorate.

As metal composition of metallic salt added to the flux of the presentinvention, there are, for example, gold, silver, copper, lead, zinc,bismuth, indium, antimony, and nickel. Especially, copper is preferred.As acid composition, there are used organic acids such as various fattyacids, and resin acids; and inorganic acids such as hydrogen halide,nitric acid, and sulfuric acid. Specifically, lead stearate, bismuthoctylate, copper naphthenate, indium acetate, silver nitrate, and zincchloride are usable. In the present invention, it is preferable to useorganic acid such as various fatty acids and resin acids, as acidcomposition. Specifically, it is preferable to use organic acid such assaturated fatty acids and resin acids whose alkyl portion has a carbonnumber of usually about 7 to 21, such as stearic acid, octyl acid,naphthenic acid, and rosin acid (including diterpene acid such asabietic acid and pimaric acid contained as main component of rosin).

The content of metallic salt is in an amount of 0.1 to 20% by weight ofthe total amount of flux. When the content is less than 0.1% by weight,it is difficult to suppress the nickel in the land surfaces fromdiffusing into solder alloy, and hence it is impossible to improve thebonding strength of solder. On the other hand, when the content is over20% by weight, insulation resistance may drop.

When metallic salt is metallic salt of organic acid, activator ispreferably organic acid that is the same as organic acid composing themetallic salt of organic acid, or organic acid having a lower aciditythan that. Usually, acidity can be evaluated by the acidity constant(electrolytic dissociation constant) Ka value and pKa value (-logKa) ofsolution. In general, acidity tends to decrease as the carbon number oforganic acid increases. Therefore, organic acid of low acidity meansorganic acid having a greater carbon number than organic acid composingmetallic salt of organic acid. For example, with respect to copperoctylate, organic acid having a greater carbon number than that, such asstearic acid, may be used as activator. The above-mentioned rosincontaining rosin acid is suitably usable as organic acid of low acidity.

If organic acid having a higher acidity than organic acid compositionmetallic salt of organic acid is used as activator, the acid maydecompose the metallic salt of organic acid, and hence such organic acidis unsuitable. Examples of suitable combinations of metallic salt oforganic acid and activator (organic acid) are copper stearate andstearic acid; and copper stearate and rosin.

The content of activator is in amount of 0.1 to 30% by weight of thetotal amount of flux. When the content is less than 0.1% by weight, thefunction of activator, namely, the active power for removing andcleaning metallic oxide on the metal surface is insufficient, so thatsolderability may be lowered. On the other hand, when the content isover 30% by weight, the film forming ability of flux decreases andhydrophilicity increases, so that corrosivity and insulation ability maybe lowered. In the case of using rosin so as to perform both functionsof resin and activator, the contents of rosin and derivative thereofmust be in an amount, within which both functions of these are notimpaired.

As solvent, there are, for example, alcohol solvents such as ethylalcohol, isopropyl alcohol, ethyl Cellosolve, butyl carbitol, and hexylcarbitol (diethylene glycol monohexyl ether); and ester solvents such asethyl acetate and butyl acetate; and hydrocarbon solvents such astoluene and turpentine oil.

The content of solvent is in an amount of 5 to 70% by weight of thetotal amount of flux. When organic solvent is less than 5% by weight,the viscosity of flux increases, and the application ability of fluxmight deteriorate. On the other hand, when organic solvent is over 70%by weight, the ratio of effective compositions (e.g., resin) as flux isreduced, so that solderability may be lowered.

The flux of the present invention can be manufactured by mixing therespective compositions as above described, and then melting whileheating. The flux of the present invention may contain othercompositions such as thixotropy agent, in addition to theabove-mentioned compositions. As thixotropy agent, there are, forexample, hydrogenated castor oil (hardened castor oil), beeswax,carnauba wax, amide stearate, ethylenebisamide hydroxystearate. Thecontent of thixotropy agent is preferably in an amount of 1.0 to 25% byweight of the total amount of flux.

Additionally, the flux of the present invention may be used togetherwith synthetic resin such as polyester resin, phenoxy resin, and terpeneresin, which have been known and used as base resin of flux. It is alsoable to add additives such as antioxidant, fungicide, and delusteringagent.

The flux of the present invention is used in cases where electrolessnickel plating is already applied to the lands of a board. Although nolimitations is imposed on the metal of the lands to which electrolessnickel plating is applied, but copper is preferred.

Although no limitations is also imposed on the type of solder alloy usedfor soldering, general tin/lead alloy solder is usable. Alternatively,there may be used so-called lead-free solder in which metal such assilver, zinc, bismuth, indium, and antimony is mixed with tin that isused as base.

In a soldering method of the present invention, the above-mentioned fluxfor soldering is printed by screen printing on a board having on itssurface copper lands to which electroless nickel plating is alreadyapplied; and solder balls are mounted on the lands and heated to havethe solder balls reflow and connect onto the lands. Solder balls arepreferably composed of lead-free solder, as above described, from thepoint of view of environmental impact. The reflow of solder balls isperformed after printing by, for example, preheating at 150 to 200° C.and heating at 170 to 250° C., or directly heating without preheating.Printing and reflow may be carried out in the atmosphere of air, or inthe atmosphere of an inert gas such as nitrogen, argon, and helium.

In the present invention, paste that is a mixture of the above-mentionedflux for soldering and solder power may be printed on a board having onits surface copper lands subjected to electroless nickel plating,followed by heating to have the paste reflow so as to form solder alloyon the lands.

Examples of the present invention will be described below. It isunderstood, however, that the examples are for the purpose ofillustration and the invention is not to be regarded as limited to anyof the specific materials or condition therein.

EXAMPLES Experimental Example 1

Lands having a diameter of 0.8 mm, in which electroless nickel platingwas applied to copper, were formed on a board, and the following fluxeswere printed individually in a thickness of 200 μm on the board, andsolder balls of Sn-37Pb having a diameter of 0.8 mm were mounted on thelands, followed by heating to have the solder balls reflow.

Flux 1: RA flux (chlorine content: 0.2%) (blank);

Flux 2: 5 wt % of lead stearate was added to Flux 1; and

Flux 3: 3 wt % of copper naphthenate was added to Flux 1

The compositions of the RA flux is as follows: WW class tall rosin 70parts by weight Hexyl carbitol 25 parts by weight Hydrogenated castoroil 4.5 parts by weight Ethylamine HC1 0.5 parts by weight

Experimental Example 2

Lands having a diameter of 0.8 mm, in which electroless nickel platingwas applied to copper, were formed on a board, and the following fluxeswere printed individually in a thickness of 200 μm on the board, andsolder balls of Sn-3.5Ag having a diameter of 0.8 mm were mounted on thelands, followed by heating to have the solder balls reflow.

Flux 4: Water soluble flux (chlorine content: 2%) (blank);

Flux 5: 2 wt % of lead stearate and 3 wt % of copper naphthenate wereadded to Flux 4; and

Flux 6: 3 wt % of bismuth octylate was added to Flux 4

The compositions of the water soluble flux is as follows: Polyamineresin 60 parts by weight Rosin EO addition product 35 parts by weightEthylamine HCl 5 parts by weight

[Experimental Example 3

Lands having a diameter of 0.8 mm, in which electroless nickel platingwas applied to copper, were formed on a board, and paste prepared bymixing 10 wt % of each of the following fluxes and 90 wt % of solderpowder of Sn-37Pb (mean particle diameter: 20 to 40 μm) was printedindividually in a thickness of 200 μm on the board, followed by heatingto have the solder powder reflow.

Flux 7: RA flux (chlorine content: 0.2%) (blank);

Flux 8: 5 wt % of lead stearate was added to Flux 7; and

Flux 9: 3 wt % of copper naphthenate was added to Flux 7 The used RAflux was the same as that used in Experimental Example 1.

[Evaluation Method]

1. Distribution States of Nickel and Phosphorous

In regard to the boards soldered in the above respective experimentalexamples, the distribution states of nickel and phosphorous in thesoldered portions were observed with an X ray microanalyzer, in order toexamine the presence or absence of nickel diffusion, and the presence orabsence of concentration of phosphorous.

2. Bonding strength

In regard to the soldered portions of the boards soldered in the aboverespective experimental examples, tension test was carried out tomeasure bonding strength. Ones whose mean strength was improved by notless than 10% than the blanks of Fluxes 1, 4, and 7 were judged as beingeffective in improving bonding strength.

3. Solderability

With a microscope at x20 magnification, the soldered portions of theboards soldered by the above respective experimental examples wereobserved to examine the wetting state of solder to the boards.

The results of the above respective tests are shown in Table 1. TABLE 1Concen- Bonding Diffusion tration Strength Solder- Flux Solder of Ni ofP (N) ability 1 Sn—37Pb presence presence 14.2 good 2 Solderball absenceabsence 16.0 good 3 absence absence 16.8 good 4 Sn—3.5Ag presencepresence 12.9 good 5 Solderball absence absence 14.8 good 6 absenceabsence 15.5 good 7 Sn—37Pb presence presence 13.2 good 8 Paste absenceabsence 16.4 good 9 absence absence 17.1 good

Subsequently, the following tests were conducted in order to confirm theinfluence exerted on solderability and preservation stability by thecombination of metallic salt added for improving the bonding strength ofsoldering, and activator used together with the metallic salt.

Examples 1 to 8, and Comparative Examples 1 to 7

[Selection of Metallic Salt and Activator]

The following materials were selected as metallic salt:

A: (C₁₇H₃₅COO)₂Cu

B: (C₇H₁₅COO)₂Cu

C: (C₂₁H₄₃COO)₂Cu

D: (C₁₇H₃₅COO)₂Pb

The following materials were selected as activator:

E: C₇H₁₅COOH

F: C₁₇H₃₅COOH

G: C₂₁H₄₃COOH

H: (CH₃)₂CHNH₂.HCl

Here, the strength of acidity can be expressed as follows: H>E>F>G.

[Preparation of Rosin Flux (RF)] WW class tall rosin 70 parts by weightHexyl carbitol 25 parts by weight Hydrogenated castor oil 5 parts byweight

Based on the above prescription, the respective materials were mixed,and this was melted while heating to 120° C., then cooled to roomtemperature, thereby preparing a viscous flux.

[Preparation of Acryl Flux (AF)] Acryl resin (Paraloid B-48N, 50 partsby weight manufacture by Rohm and Haas Company) Hexyl carbitol 45 partsby weight Hydrogenated castor oil 5 parts by weight

Based on the above prescription, the respective materials were mixed,and this was melted while heating to 120° C., then cooled to roomtemperature, thereby preparing a viscous flux.

[Preparation of Copper Salt and Activator Mixed Flux]

The previously selected metallic salt and activator, and the base fluxso prepared (the above-mentioned RF or AF) were mixed, and this wasmelted while heating to 100° C., then cooled to room temperature. Thetypes and mixing ratios (% by weight) of the used base fluxes, metallicsalts, and activators are shown in Table 2. TABLE 2 Unit: wt. BaseFluxes Metallic Salt Activator Mixing Mixing Mixing Type Ratio (wt.)Type ratio (wt %) Type Ratio (wt.) Example 1 RF 90 A 10 — — Example 2 RF88 A 10 F 2 Example 3 RF 88 A 10 G 2 Example 4 RF 88 B 10 E 2 Example 5RF 88 C 10 G 2 Example 6 RF 88 D 10 F 2 Example 7 AF 88 A 10 F 2 Example8 AF 88 A 10 G 2 Comparative RF 100 — — — — Example 1 Comparative RF 88A 10 E 2 Example 2 Comparative RF 88 A 10 H 2 Example 3 Comparative RF88 C 10 F 2 Example 4 Comparative AF 90 A 10 — — Example 5 ComparativeAF 88 A 10 E 2 Example 6 Comparative AF 88 A 10 H 2 Example 7[Evaluation Method]1. Measurement of Insulation Resistance

On a board in which lands were disposed in a tandem pattern, the fluxobtained in each of Examples and Comparative Examples was printedoverall on the board in a thickness of 100 μm, followed by reflow at amaximum temperature of 250° C. The fluxes used in the measurement ofinsulation resistance were of two types, namely, the flux immediateafter manufacturing, and the flux left for one month at 40° C. Afterreflow, based on the test method of JIS Z 3197, the board was left for1000 hours at a temperature of 85° C. in an atmosphere having a relativehumidity of 85%, while applying a voltage of DC50V to the board. Then,the insulation resistance between the lands was measured.

2. Measurement of Bonding strength

Electroless nickel plating was applied to copper lands on a board, andgold plating was further applied thereto, thereby forming lands having adiameter of 0.4 mm. On the board, the flux of each of Examples andComparative Examples (immediately after manufacturing) was printed insolid state in a thickness of 100 μm.

Then, on the lands, Sn-3.5Ag-0.5Cu solder balls were mounted and allowedto reflow and connect onto the lands. Subsequently, this board wasimmersed in and cleaned with an ultrasonic washer filled withbutylcarbitol solution of 60° C., in order to remove the flux.

Next, with a DAGE-SERIES-4000P, manufactured by DAGE Incorporation, thebonding strength of the solder balls was measured (heated bump pullstrength). The measurement was made at 30 points, respectively, and itsmean value was employed as bonding strength, and its minimum value asminimum bonding strength.

3. Wettability Evaluation

On a copper plate subjected to oxidation at 150° C. for one hour, 0.025g of each flux (immediately after manufacturing) of Examples andComparative Examples in Table 1 was mounted, and Sn—Pb solder ballshaving a diameter of 1 mm were mounted thereon, followed by heating witha hot plate of 230° C. for one minute. Thereafter, the height of thesolder was measured, and the wetting and spreading rate was calculatedfrom the following equation. Wetting and spreading rate is to evaluatethe active power of activator, and a higher wetting and spreading rateindicates a higher active power of activator.

Wetting and Spreading Rate=(1−Height of Solder)×100

The test results of these are shown in Table 3. TABLE 3 InsulationResistance (Ω) Flux immediate Flux left Bonding Minimum Wetting andafter for one month strength Bonding Spreading manufacturing at 40° C.(N) strength (N) rate (%) Example 1 1.8 × 10¹² 1.5 × 10¹² 18.5 15.5 78%Example 2 2.0 × 10¹² 1.8 × 10¹² 19.0 14.8 87% Example 3 1.5 × 10¹² 1.7 ×10¹² 18.0 16.0 85% Example 4 1.2 × 10¹² 1.0 × 10¹² 17.8 14.0 84% Example5 1.4 × 10¹² 1.5 × 10¹² 18.3 14.7 81% Example 6 1.7 × 10¹² 1.5 × 10¹²18.6 15.8 86% Example 7 1.5 × 10¹² 1.8 × 10¹² 18.2 14.3 82% Example 81.8 × 10¹² 1.3 × 10¹² 17.9 15.0 83% Comparative 3.0 × 10¹² 2.0 × 10¹²15.1 6.3 76% Example 1 Comparative 1.8 × 10¹¹ <1 × 10⁷  17.0 13.9 85%Example 2 Comparative 1.5 × 10¹⁰ <1 × 10⁷  16.8 14.2 87% Example 3Comparative 2.0 × 10¹¹ <1 × 10⁷  17.8 14.5 82% Example 4 Comparative 2.0× 10¹² 1.8 × 10¹² 16.5 14.5 65% Example 5 Comparative 1.3 × 10¹¹ <1 ×10⁷  17.2 14.0 80% Example 6 Comparative 1.2 × 10¹⁰ <1 × 10⁷  16.4 13.882% Example 7

As shown in Table 3, the fluxes of Examples 1 to 8 do not decreaseinsulation resistance after being left at 40° C. for one month. To thecontrary, in Comparative Examples 2, 3, 4, 6, and 7, the stability ofmetallic salt was poor, and the drop in insulation resistance wasobserved in use of the flux after being left at 40° C. for one month.

All of the fluxes of Examples 1 to 8 maintain a high bonding strength.Whereas in Comparative Example 1, the mean bonding strength was lowbecause it does not contain metallic salt, and some points indicate anextremely low bonding strength. Comparative Example 5 indicates a lowwetting and spreading rate due to a low active power.

1. A flux for soldering used when soldering is performed to a boardsubjected to electroless nickel plating, said flux containing resinhaving film forming ability, activator, and solvent, and furthercontaining metallic salt in an amount of 0.1 to 20% by weight of a totalamount of flux.
 2. The flux according to claim 1, wherein said metallicsalt is salt of metal selected from gold, silver, copper, lead, zinc,bismuth, indium, antimony, and nickel.
 3. The flux according to claim 1,wherein said metallic salt is metallic salt of organic acid, and saidactivator is organic acid that is same as organic acid composing saidmetallic salt of organic acid, or organic acid having a lower aciditythan that.
 4. The flux according to claim 2, wherein said metallic saltof organic acid is copper salt of organic acid.
 5. The flux according toclaim 3, wherein said metallic salt of organic acid is metallic salt ofsaturated fatty acid whose hydrocarbon group except for carboxyl group,has a carbon number of 7 to
 21. 6. The flux according to claim 3,wherein said activator is rosin.
 7. The flux according to claim 1,wherein said resin having film forming ability is rosin or acrylicresin.
 8. The flux according to claim 1, wherein said resin having filmforming ability is rosin, and said rosin also functions as saidactivator.
 9. The flux according to claim 1, containing no halogencompound.
 10. A soldering method including: printing a flux forsoldering according to claim 1 on a board having on its surface copperlands subjected to electroless nickel plating; mounting solder balls onsaid lands; and heating to have said solder balls reflow and connectonto said lands.
 11. The soldering method according to claim 10, whereinsaid solder balls are lead-free.
 12. A soldering method including:printing paste that is a mixture of a flux for soldering according toclaim 1 and solder powder, on a board having on its surface copper landssubjected to electroless nickel plating; and heating to have said pastereflow so as to form solder alloy on said lands.
 13. A printed circuitboard in which solder is connected by a soldering method according toone of claims 10 to 12.